1. Field of the Invention
The present invention relates generally to an apparatus for moving an optical system, and more particularly to an apparatus for moving an optical system built in an optical information recording/reproducing apparatus for recording information on an optical recording medium and reproducing information from the medium by radiating a light beam on the medium.
2. Description of the Related Art
FIG. 1 shows a conventional apparatus for moving an optical system. An optical head 40 for optically reproducing information from an optical recording medium (an optical disc) and for recording information on the medium comprises an optical chassis 42 containing an optical system, and an objective lens actuator 44 situated movably on the chassis 42 for directing a light beam onto the recording medium and picking up the beam from the medium. An objective lens 2 for converging a light beam onto the information recording medium is mounted on the objective lens actuator 44. The lens 2 is movable over a short distance in the focusing direction (Z-direction) by means of a focusing actuator system housed in the objective lens actuator 44, and also movable over a short distance in the traking direction (X-direction) by means of a lens actuator system housed in the objective lens actuator 44.
The chassis 42 contains an optical system comprising a guiding optical system for shaping and collimating a light beam output from a light source and transmitting the beam to the objective lens 2, and a detecting optical system for detecting the beam reflected from the optical disc. A guide bushing 46 is fixed on each of both sides of the optical chassis 42 (in FIG. 1 only the bushing on one side is shown). A guide shaft 48 extending in the tracking direction is inserted into each guide bushing 46. (In FIG. 1, part 50 where the guide shaft 48 is inserted into the bushing 46 is enlarged.) Thus, the optical head 40 is supported to be movable in the X-direction. The X-directional movement of the optical head 40 is effected by driving an arm 60 coupled to the chassis 42 by means of an electromagnetic driving system 62. The electromagnetic driving system 62 comprises a magnetic circuit 64 and a drive coil 66. The magnetic circuit 64 includes a yoke 64a and a permanent magnet 64b. The drive coil 66 is wound around the arm 60 and inserted into the magnetic circuit 64. The coil 66 is kept out of contact with the yoke 64a or permanent magnet 64b. A driving force or a Lorentz force is created on the basis of the direction of electric current flowing in the drive coil 66 and the direction of magnetic flux of the permanent magnet 64b. The driving force is applied to the optical head 40 through the arm 60, and the head 40 is moved along the guide shaft 48.
In relation to the above optical system moving apparatus, suppose that an optical recording medium (optical disc D) having a recording region with a radial length R is accessed by the objective lens 2, as is shown in FIG. 2. In general, the seek distance of the lens 2 is divided into a small-stroke distance L1, a medium-stroke distance L2 and a large-stroke distance L3. In FIG. 2, the small-stroke distance L1 corresponds to a very short seek distance (e.g. 0.3 mm or less in a general optical disc) which is almost equal to the amount of displacement of the disc D or to several track pitches. The large-stroke distance L3 corresponds to the radial length of the recording region R of the disc D, i.e. the full stroke of the head 40. The medium-stroke distance L2 corresponds to a seek distance which is between the seek distances of the small-stroke distance L1 and the large-stroke distance L3. Regarding FIG. 2, it should be noted that the strokes L1, L2 and L3 are shown only to indicate the comparison therebetween. The beginning points and end points of the small-stroke distance L1 and the medium-stroke distance L2 on the disc D are not exactly shown.
Regarding the conventional optical system moving apparatus shown in FIG. 1, when the small-stroke distance L1 is accessed, the lens actuator system is driven to move the objective lens 2. When a distance exceeding the small-stroke distance L1 is accessed, the entire body of the optical head 40 is driven by the electromagnetic drive system 62. In this apparatus, when the information recording medium D is random-accessed by the objective lens 2, the distance which is most frequently accessed is about 1/3 to 1/6 of the full stroke, in particular, about 1/3 of the large-stroke distance L3 when the optical recording medium is disk-shaped. The 1/3 of the large-stroke distance L3 corresponds to the medium-stroke distance L2. The access over this distance is carried out by the electromagnetic drive system 62 in the conventional apparatus. In this case, however, since the weight of the optical head 40 which is moved by the electromagnetic drive system 62 is heavy, the drive acceleration cannot be increased. Consequently, the access time necessary for accessing the middle-stroke distance L2 increases. In addition, it is not possible to shorten the average seek time period which is calculated by averaging seek times T1, T2 and T3 for seeking the small-, middle- and large-stroke distances L1 to L3. Because of these problems in the prior art, the increase in recording/reproducing speed with use of the optical head is limited.